Osmolyte regulation in abiotic stress

Abstract

To withstand osmotic stress induced by salinity, drought or extreme temperatures, all organisms have evolved a machinery to synthesize metabolites, termed "compatible solutes" or "osmo-protectants", which help in raising the osmotic pressure and thereby maintaining both the turgor pressure and the driving gradient for water uptake. In addition, these compounds also help in maintaining the structural integrity of enzymes, membranes and other cellular components during the stress regime. Of special importance among these metabolites is nitrogen containing compounds (e.g., quaternary amino compounds and proline) and hydroxyl compounds (e.g., polyols and oligosaccharides). These compounds are distributed throughout the biological kingdom and are generally products of stress-induced pathway extensions, although normal metabolites such inositols may also act as osmolytes. Chemically, different osmolytes function through a common mechanism of stabilization of proteins under stress or by osmotic adjustments, and these mechanisms seem to be universal among the biological system. Over-expression of genes for the synthesis of different osmolytes in transgenics enables the plants to cope better with the stress due to higher accumulation of the concerned osmolytes. However, in several cases, such as trehalose and inositol, the accumulation is far below the required amount and it is conjectured that these metabolites might function in a manner unrelated to their osmolyte role and are hence more involved in the general growth and development of the plants under abiotic stress conditions.